1. Field of the Invention
This invention relates generally to methods for formatting disks and in particular to a method of remapping a defective disk for embedded servo operation that enhances yields and eliminates spurious seeks to auxiliary tracks.
2. Description of Related Art
Typically, a disk drive contains one or more circular planar disks that are coated on each side with a magnetic medium. The disk or disks are mounted on a spindle that extends through the center of each disk so that the disks may be rotated at a predetermined speed, usually about 3600 rpm. Typically, one read/write head is associated with each side of the disk that is coated with a magnetic medium. The read/write head flies a small distance above the disk surface as the disk rotates. The read/write head, in response to signals from electronics associated with the disk drive, writes data at a predetermined location in the magnetic medium. Similarly, the read/write head, in response to other signals from the disk drive electronics, reads the data stored at a predetermined location.
The configuration of the data on the magnetic surface is instrumental in the operation of the disk drive. Data are recorded by the read/write head in concentric circular tracks on the disk. Typically, each track is segmented into one or more parts that are referred to as sectors. Thus, the disk drive must move the read/write head radially across the disk surface to locate the track for reading or writing data and then must follow that track circumferentially until the desired sector passes under the read/write head. Hence, the read/write head is positioned at a predetermined radial and circumferential position over the disk surface.
In a disk drive, each read/write head is usually affixed by an arm to an actuator. In a closed-loop disk drive, a servo system is used to move the actuator. Many different servo systems have been developed for use in hard disk drives. In an embedded servo system, a servo pattern contained in a servo field at the start of each sector was used to determine the radial and circumferential position of the read/write head relative to the disk. The servo information that was read by the read/write head was provided to the disk drive control loop electronics which in turn generated signals to reposition the read/write head as necessary based on that servo information. The servo pattern also typically contained synchronization signals for the disk drive electronics.
The data in the servo field was pre-recorded on the disk and the disk drive electronics were designed to prevent writing user data over the servo field data. Since the pre-recorded data in the servo fields were used to position the read/write heads and for synchronization, all the information in each servo field had to be free of defects. Unfortunately, the manufacturing process used to place the magnetic coating on the disk often fails to produce a defect free surface. Therefore, there is a likelihood that when the servo field data is recorded on the disk, a defect occurs in the location of one or more of the servo fields.
Defects in pre-recorded servo field information are typically grouped into three classes:
Class 1: Minor defects that are either temporary or permanent where the servo information is corrupted but the track containing the corrupted servo information is usable; PA0 Class 2: Intermediate defects that affect a single track and are permanent where the servo information is corrupted and the single track is not usable; and PA0 Class 3: Large defects that are permanent and extend radially across more than one track where the servo information is corrupted and the tracks with the corrupted servo are not usable. PA0 generating a list containing an entry for each absolute track number containing a defect wherein each entry is the absolute track number; and PA0 adjusting a seek length to a target track number by increasing the seek length by one track for each entry in the list that has an absolute track number greater than or equal to the target track number. PA0 a) checking the list to determine whether there is an entry in the list to process; PA0 b) positioning a transducer upon step a) determining that there is not an entry left to process; PA0 c) retrieving an absolute track number entry from the list upon step a) determining there is an entry left in the list to process; PA0 d) comparing the target track number with the retrieved absolute track number entry; PA0 e) adding one to the target track number upon step d) determining that the target track number is equal to or greater than the retrieved absolute track number entry; PA0 f) branching to step a) upon completion of step e); and PA0 g) branching to step a) upon step d) determining that the target track number is less than the retrieved absolute track number entry.
FIG. 1 is a representation of a disk 100 that includes concentric circular tracks 101-1, 101-2, 101-3. Regions 102-1, 102-2, 102-3 are embedded servo fields that each contain servo data. Each region 102-1, 102-2, and 102-3 contain n servo fields, i.e., one servo field in each of the n concentric data tracks on the disk. Those skilled in the art will appreciate that FIG. 1 is only a partial representation of disk 100, and that this partial representation is sufficient to adequately represent the general features of disk 100.
As illustrated in FIG. 1, region 102-2 has a defect 110 in track 101-1. Centerline 130 of region 102-2 is oriented at angle x.degree. from line 140, which coincides with the leading edge of region 102-1, where centerline 130 and line 140 intersect at center 120 of disk 100.
If defect 110 was a Class 1 defect, prior art methods typically used cyclic redundancy codes to correct such defects in the data fields. Therefore, if defect 110 was moved into the data region, the track containing the defect would still be usable. Class 1 defects in the servo field were corrected typically using hardware information recovery techniques, such as similar rereads.
If defect 110 was a Class 2 defect, defect 110 was typically handled by repositioning the servo field in which defect 110 occurred. In one method, regions 102-1, 102-2, and 102-3 were angularly repositioned. For example, in FIG. 2, the pre-recorded servo information in regions 102-1, 102-2, and 102-3 has been angularly rotated so that defect 110 is no longer contained in region 102-2.
The angular rotation method is usually effective if the distribution of defects is random and the absolute quantity of defects is small as is the case in disk drives with a low areal density, i.e., disk drives with a low information capacity.
However, if the absolute quantity of defects is not small, there is a good likelihood that after angular rotation, another defect is contained in one of the servo field regions. For example, in FIG. 3, after angular rotation for Class 2 defect 110, defect 310 is located in region 102-1. Moreover, with increased recording density more sector fields per track are employed and narrower tracks are utilized. The result is that a greater number of sector field defects are discovered.
A second prior art strategy to cope with sector field defects was to utilize auxiliary tracks located adjacent to the crash stop zones. The auxiliary tracks were used for utility and diagnostic purposes and a small number of auxiliary tracks were available to map as replacements for tracks with permanent defects in sector information fields.
In FIG. 4, tracks in zone 410 are used for data storage. Track 101-1 has defect 110 in region 102-2. In the second prior art method, track 101-1 is mapped to track 401-1 in zone 420. Unfortunately, the quantity of tracks in zone 420 is limited and is determined by the crash stop physical tolerances.
Further, using replacement tracks, such as track 401-1, in zone 420 requires additional undesirable seek operations and latency delays which act to reduce disk drive performance. For example, in a sequence that accesses in order tracks 101-3, 101-1, and 101-2, track 101-3 is located in a first seek. Normally, the second seek would be a two track seek. However, defect 110 caused track 101-1 to be remapped to track 401-1. Thus, a longer seek is required, i.e., a seek from track 101-3 to track 401-1 instead of a seek from track 101-3 to track 101-1. Similarly, the seek from track 101-1 to track 101-2 would have been a single track seek, but now the seek is from track 401-1 to track 101-2. Hence, the remapping of track 101-1 to track 401-1 introduces two large seek discontinuities in accessing data that are expected to be in adjacent tracks.
The above prior art methods were utilized primarily for Classes 1 and 2 defects only. A Class 3 defect is a previously unaddressed problem affecting miniature disk drives utilizing high areal densities, i.e., those that use high linear recording densities, e.g., densities in the range of 25,000 bits per inch (BPI) or more simultaneously with high track density, e.g., 2000 track per inch (TPI) and higher. In these miniature disk drives, the tracks are so narrow and so close together that a single physical defect can simultaneously affect a number of radially adjacent tracks. The high linear recording densities increase the probability of discovering even more defects.
As the number of defects increases, the prior art methods for compensating for the defects will probably be ineffective. Therefore, the disk drive must be disassembled and the defective disk replaced with another disk. Disassembly and repair is expensive. Therefore, a better method for handling disk defects is needed for increased areal recording density and, in particular, for miniature disk drives.